U.S. Pat. No. Re. 27,617 (Olsen) teaches a process of making a louvered light control film by skiving a billet of alternating layers of plastic having relatively lower and relatively higher optical densities. Upon skiving the billet, the pigmented layers provide light collimating louver elements, which, as illustrated in the patent, may extend orthogonally to the resulting louvered plastic film. U.S. Pat. No. 3,707,416 (Stevens) teaches a process whereby light collimating louver elements may be canted with respect to the surface of the louvered plastic film to provide a film which collimates light in a direction other than perpendicular to the surface of the film. U.S. Pat. No. 3,919,559 (Stevens) teaches a process for attaining a gradual change in the angle of cant of successive light collimating louver elements.
Among the uses for such louvered plastic films are lenses and goggles as shown in U.S. Pat. No. 3,791,722 (Ahlberg et al), to be worn where high levels of illumination or glare are encountered. The film may also be used for a transparent covering for a backlighted instrument panel, such as the instrument panel of an automobile, to minimize reflections from being cast onto the windshield. A louvered plastic film may also be used to give a black and white photographic negative the appearance of a positive made from the negative, as taught in U.S. Pat. No. 3,653,138 (Cooper).
U.S. Pat. No. 3,922,440 (Wegwerth et al) points out that because louvered plastic films "are thin sheet materials: (1) they are not by themselves capable of structurally withstanding extreme stresses and (2) they are subject to distortion from physical stress and temperatures" (col. 1, lines 19-22). Furthermore, the skiving by which the louvered plastic films are produced results in irregular surfaces which seriously limits the optical quality of the film. Typically such films are, for practical purposes, opaque. Accordingly, as in Example 1 of that patent, the louvered plastic film usually is laminated under pressure between two clear plastic films of a material such as cellulose acetate butyrate, the material usually used in making louvered plastic films. Typically, the louvered plastic film is skived from the billet to a thickness between 0.2 and 0.4 mm, each of the outer plastic films has a thickness of between 0.15 and 0.3 mm. The thickness of the skived film will control the permitted view angle, with a thicker film providing a narrower angle. Wegwerth's process of laminating louvered plastic films between two clear films requires an expensive press which is also expensive to operate. This is in part from the need to distribute heat uniformly and in part from the need to apply pressure with precision. Because the resulting laminates can not be larger than the platens of the press in which they are laminated, the press must be sufficiently large to produce the required size thus increasing the expense of the press.
Commonly assigned copending application Ser. No. 908,990 teaches an alternative to the Wegwerth method. The alternative method includes the steps of (1) coating the skived louvered plastic film with a solventless monomer composition which polymerizes to an adhesive state upon exposure to ultraviolet radiation, (2) overlaying the monomer composition with a plastic film, and (3) exposing the coating to ultraviolet radiation to polymerize the composition to an adhesive state. After polymerization the plastic liner which was placed over the monomer composition may be left in place to serve as protection for the louvered plastic film, or may be removed, leaving the polymerized composition exposed to act as a pressure-sensitive adhesive for bonding the film to other materials.
The alternative method is advantageous over the Wegwerth method in that it can be used in a continuous process rather than only in a batch process as is the case with the Wegwerth method. Furthermore, the alternative method does not require the expensive press and the precise application of heat and pressure which the Wegwerth method requires. The alternative method is disadvantageous, however, because the monomer compositions which polymerize to an adhesive state polymerize slowly. Thus, for use in a continuous process, either a very long radiation chamber is required or the film must move through the radiation chamber very slowly. In either case the expense of producing the film is again increased. A disadvantage which is common to the alternative method and the Wegwerth method lies in the fact that the protective liners and skived louvered plastic film produce a laminate which is relatively stiff. In some applications, such as when the louvered plastic film is to cover a touch sensitive display screen, a more flexible film is desirable.